Crystal structure and characterization of the sulfamethazine–piperidine salt

The piperidinium–sulfamethazinate salt was prepared by the solvent-assisted grinding method. The ions are connected by N—H+⋯O and N—H+⋯N interactions. The self-assembly of sulfamethazinate anions displays the amine–sulfa C(8) motif. The supramolecular architecture of the salt revealed the formation of interconnected supramolecular sheets.


Introduction
Sulfonamides are antimicrobial drugs used for the treatment of human and veterinary bacterial infections, and act by inhibiting the enzyme dihydropteroate synthase, a key enzyme involved in folate synthesis (Ovung & Bhattacharyya, 2021). The chemical structure of sulfonamides includes SO 2 , NH and NH 2 groups capable of acting as hydrogen-bond donors and acceptors, and also arene rings capable of forming �-interactions, which make them suitable supramolecular building blocks for use in crystal engineering for the formation of pharmaceutical cocrystals (Caira, 2007).
Pharmaceutical cocrystals are crystalline materials composed of an active pharmaceutical ingredient (API) and a cocrystal coformer which remain together in the crystalline lattice principally via hydrogen-bond interactions. Pharmaceutical cocrystallization offers the possibility of obtaining new solid forms of APIs and improving poor physicochemical properties (Bolla & Nangia, 2016).
Piperidine (PPD) is a heterocyclic amine that possesses an N-H group able to act as a hydrogen-bond donor. Combination with pharmaceutical ingredients gives rise to the formation of cocrystals (with curcumin; Sanphui & Bolla, 2018) or salts [with diclofenac (Fini et al. 2012) and sulfapyridine (Pratt et al., 2011)]. The formation of cocrystals or salts can be predicted (not exactly) using the �pK a criteria from [pK a (base) -pK a (acid)]. When the value of �pK a is greater than 3, salt formation occurs, and when the value of �pK a is less than 0, cocrystal formation occurs. �pK a values between 0 and 3 do not give clear information about the cocrystal/salt preference (Kumar & Nanda, 2018). We report here a crystallization study between sulfamethazine (pK a = 7.40; Zhang et al., 2016) and piperidine (pK a = 11.10; Luna et al., 2016) (Fig. 1), producing the PPD + ·SUL À salt, I (�pK a = 3.7) (Scheme 1) by solvent-assisted grinding and solvent evaporation. The solid-state characterization was performed by IR spectroscopy (IR), powder X-ray diffraction (PXRD), solid-state nuclear magnetic resonance ( 13 C NMR) spectroscopy, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The crystal structure was obtained by single-crystal X-ray diffraction.

Synthesis and crystallization
Sulfamethazine and piperidine were purchased from Aldrich. Dichloromethane and ethanol were purchased from Química Mayer. All reagents were used as received.
Sulfamethazine (0.3 g, 1.077 mmol) and piperidine (0.106 ml, 1.077 mmol), in a 1:1 molar ratio, were placed in a mortar. Before grinding, 0.5 ml of dichloromethane was added. The mixture was then ground with a pestle for 3 min. After the grinding time, the dichloromethane was evaporated and the powder was collected in the centre of the mortar. The cycle of adding 0.5 ml of dichloromethane and grinding for 3 min was repeated three more times until a grinding time of 12 min was reached. The polycrystalline ground powder of I was collected and stored in a glass vial. Single crystals suitable for X-ray diffraction were obtained from a solution of I in ethanol left to evaporate at room temperature.

IR spectroscopy
The IR spectra of solid powders of SUL and PPD, the polycrystalline powder of I and the single crystal of I were acquired in a Bruker Tensor 27 spectrophotometer equipped with an attenuated total reflectance (ATR) system accessory (16 scans, spectral range 600-4000 cm À 1 , resolution 4 cm À 1 ).

Powder X-ray diffraction
Powder X-ray diffraction (PXRD) patterns of SUL, PPD and the polycrystalline powder of I were recorded on a PANalytical X'Pert PRO diffractometer with Cu K� 1 radiation (� = 1.5405 Å , 45 kV, 40 mA) from 2.02 to 49.93 � in 2�.

Solid-state 13 C NMR
Cross-polarization/magic angle spinning (CP/MAS) 13 C NMR experiments of the solid powders of SUL and the polycrystalline powder of I were performed on a Bruker 400 Avance III ( 13 C, 100 MHz) instrument at 25 � C. 4 mm bullettype Kel-F zirconia rotors were used (containing about 100 mg of the sample). The spinning rate and acquisition time were 8 kHz and 32 ms, respectively. The recycle time of the pulse was 3 s. The adamantane signal was used as the external reference (� = 38.48 ppm).

Thermal analysis
Differential scanning calorimetry (DSC) measurements were obtained on a TA Instruments Q100 instrument. Samples placed in aluminium pans were heated from 25 to 255 � C under a nitrogen atmosphere at a rate of 10 � C min À 1 . Thermogravimetric analysis (TGA) was performed on a TA Instruments SDT Q600 instrument. Samples placed in aluminium pans were heated from 25 to 315 � C under a nitrogen atmosphere at a rate of 10 � C min À 1 . Heating of I was performed in a VelaQuin 9053A oven at 185 � C for 1 h (arbitrary time).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The H atoms of amine N-H groups were located in a difference map and refined isotropically with U iso (H) = 1.2U eq (N). H atoms attached to C atoms were placed in geometrically idealized positions and refined as riding on their parent atoms, with C-H = 0.95-0.99 Å and U iso (H) = 1.2U eq (C) for aromatic and methylene groups, and 1.5U eq (C) for methyl groups.

Solid-state characterization of salt I
The formation of the new solid phase of salt I was evidenced by IR spectroscopy because the IR spectrum was different from those of the starting materials PPD and SUL, showing shifts in the N-H and SO 2 bands ( Fig. 1), indicating the formation of intermolecular interactions [the IR spectra of SUL and PPD were assigned according with Yang et al. (2005) research papers and Gü llü og lu et al. (2007), respectively]. The distinctive bands in the IR spectra of the polycrystalline powder of I and the single crystal of I are those at 3076 and 3073 cm À 1 , respectively, belonging to the piperidinium N-H + group (Silverstein et al., 1991); also, the SO 2 band at 1114 cm À 1 in the polycrystalline powder and the single crystal of I, which is shifted to a lower wavenumber with respect to the starting material SUL (1145 cm À 1 ), indicated deprotonation of the sulfonamide group, as observed in the formation of the benzamidinium sulfamerazinate salt (Kamali et al., 2015) and in the formation of metallic complexes of SUL with silver and copper (Tailor & Patel, 2015). The N-H bands of SUL were also shifted from 3441 and 3339 to 3452 and 3351 cm À 1 in the polycrystalline powder of I, and to 3451 and 3350 cm À 1 in the single crystal of I (Fig. 1).
The experimental PXRD pattern of SUL matched well with the simulated pattern obtained from Mercury (Macrae et al., 2020) for the Cambridge Structural Database (CSD; Groom et al., 2016) refcode SLFNMD01 (Basak et al., 1983). The PXRD pattern of the polycrystalline powder of I was different from the PXRD pattern of SUL and matched well with the simulated pattern of crystal I obtained from Mercury (Fig. 2). The complete transformation of the starting components into the new solid phase of I was evidenced by the absence of the diffraction peaks at 2� = 9.4, 15.3, 18.6, 24.7 and 26.5 � research papers  (Agilent, 2013), SHELXT2018 (Sheldrick, 2015a), SHELXL2018 (Sheldrick, 2015b) and ORTEP for Windows (Farrugia, 2012).

Figure 2
Powder X-ray diffractograms of (a) SUL, (b) the polycrystalline powder of I and (c) the simulated pattern for I.

Figure 3
The solid-state 13 C NMR spectra of (a) the polycrystalline powder of I and (b) SUL.

Figure 1
IR spectra of (a) PPD, (b) SUL, (c) the polycrystalline powder of I and (d) crystal I.
belonging to the starting material SUL in the PXRD pattern of the polycrystalline powder of I, and the appearance of new diffraction peaks at 2� = 11.1, 12.2, 13.2, 14.5, 20.9, 22.0 and 23.1 � . The polycrystalline powder of I was characterized by 13 C CP/MAS NMR spectroscopy and each signal represents a chemically different C atom. The 13 C NMR spectrum of SUL was assigned according to Fu et al. (2016) and Grossjohann et al. (2015), and was used to assign the spectrum of the polycrystalline powder of I. The 13 C NMR spectrum of I contained the signals for both SUL and PPD, and most of the 13 C NMR signals were shifted with respect to the 13 C NMR spectrum of SUL due to the change in the chemical environment as a consequence of the formation of the salt (Fig. 3). Evidence of the deprotonation of SUL was observed by the shift to a higher frequency of the C7 signal from 155.9 ppm in SUL to 164.6 ppm in I in a similar way to when SUL is deprotonated to form metallic complexes (Hossain et al., 2007). A similar case is observed when saccharin is deprotonated to form salts with fluoroquinolones, since the 13 C NMR signal of the carbonyl C atom (next to the negatively charged N atom) is shifted from 164.0 to 172-173 ppm after deprotonation (Romañ uk et al., 2009). A comparison of the C7 chemical shifts, obtained from solid-state 13 C NMR spectroscopic analysis reported for cocrystals of SUL in the amidine form, and cocrystals and salts in the imine form (Fig. 4), revealed that in the amidine form, the C7 (C-NH) signal appeared at 155.2 ppm in the sulfamethazine-4-aminosalicylic acid cocrystal (similar to free SUL) (Grossjohann et al., 2015), while in the sulfamethazine-saccharin cocrystal, where SUL adopts the imine form, the C7 (C N) signal was shifted to a lower frequency, appearing at 152.7 ppm due to shielding caused by the formation of the C N bond. In the sulfamethazinium saccharinate imine salt, the C7 signal (C NH + ) appeared at 150.2 ppm (shifted to a lower frequency), showing greater shielding due to the protonation of the C N bond (Fu et al., 2016) (Fig. 4). On the other hand, when amidine SUL is deprotonated as in I, the C7 (C-N À ) signal appears at a higher frequency with respect to free SUL, because it is deshielded as a consequence of the negatively charged nitrogen effect. Whole assignments of the 13 C NMR signals are included in Table 2.
The thermal properties of the single crystal of I were investigated by TGA/DSC. The DSC plot of crystal I showed three endothermic peaks (Fig. 5). Considering that the crystal Chemical shift of the C7 13 C NMR signal in the different forms of SUL.

Figure 5
(a) The DSC curve of crystal I, (b) the TGA curve of crystal I and (c) the IR spectra of the polycrystalline powder of I before and after heating.
structure of I is composed of only PPD and SUL, the peak at 170.19 � C is assigned to a solid-solid transition before the evaporation of PPD, the peak at 180.42 � C is assigned to the evaporation of PPD and the peak at 198.25 � C is attributed to the melting of SUL (Singh & Baruah, 2019). The TGA plot showed a weight loss of 21.99% at 185 � C, corresponding to the loss of PPD, as suggested by the DSC curve (Fig. 5). The mass loss after the melting of SUL is attributed to the degradation of SUL. To confirm the loss of PPD, a 100 mg sample of the polycrystalline powder of I was heated at 185 � C for 1 h, then an IR spectrum was recorded and compared with that obtained at room temperature (25 � C). The IR spectrum of I obtained at 185 � C is similar to the IR spectrum of pure SUL, confirming the loss of PPD (Fig. 5).

Crystal structure of I
Salt I crystallized in the space group P2 1 /n with one SUL À anion and one PPD + cation in the asymmetric unit (Z = 4) (Fig. 6) connected by an N-H + � � �O S (N4-H4D� � �O2) hydrogen bond. The crystal structure of salt I showed deprotonation of the sulfonamide N atom of SUL and protonation of the amine group of PPD (to form the piperidinium group), as predicted by the �pK a criteria. The SUL À anion adopts a V shape, with a C1-S1-N5-C7 torsion angle of À 61.39 (11)  The asymmetric unit of I, showing the atom numbering. Displacement ellipsoids are drawn at the 50% probability level. Dashed lines represent hydrogen bonds.

Figure 7
The hydrogen-bond patterns adopted in the self-assembly of SUL.  and its cocrystals and salts in the amidine form, revealed that in the self-assembly of SUL molecules, four supramolecular patterns are preferred (three involving amine-sulfa interactions and one involving a sulfa-sulfa interaction) (Fig. 7). In salt I, the SUL À anion adopts the sulfa-amine C(8) pattern (Bernstein et al., 1995) formed by the N1-H1D� � �O1 i hydrogen bond, producing a supramolecular tape running along the b axis. Hydrogen-bond details and symmetry codes of crystal I is given in Table 3. The two-dimensional supramolecular array is formed by the interlinking of C(8) SUL À tapes with the PPD + protons (N4-H4D� � �O2 and N4-H4E� � �N2 iii ; Table 3), giving rise to a supramolecular sheet extended along the ab plane (Fig. 8). Supramolecular sheets are linked by N1-H1E� � �O1 ii and N1-H1D� � �O1 i hydrogen bonds involving two SUL À anions and two PPD + cations, showing an R 2 4 (8) hydrogen-bond motif in a similar manner to the sulfamethazine-fumaric acid cocrystal (Fig. 8) (Ghosh et al., 2011).
The crystal structure of pure SUL adopts the sulfa-sulfa C 2 2 (4) pattern (Fig. 7) and, after grinding, SUL transfers a proton to PPD (evidenced by the shift of the C7 signal in the 13 C CP/MAS NMR spectrum and the appearance of the band at 3067 cm À 1 in the IR spectrum) to form the PPD + ·SUL À salt, I, displaying the sulfa-amine C(8) motif, changing the hydrogen-bonding pattern (evidenced by the shifts in the IR bands) and the chemical environment (shifting the 13 C NMR signals). Heating I at 185 � C leads to the loss of PPD (according to DSC/TGA information and IR spectra) and the remaining SUL returns to the sulfa-sulfa C 2 2 (4) pattern before melting at 198.25 � C.

Conclusions
The salt piperidinium sulfamethazinate, PPD + ·SUL À , I, was obtained by solvent-assisted grinding. Proton transfer was confirmed by IR spectroscopy, solid-state 13 C NMR spectroscopy and single-crystal X-ray diffraction. The complete transformation of the starting material into the new crystalline phase was confirmed by PXRD analysis. The IR spectra and the PXRD patterns of the polycrystalline powder and the single crystal of I matched well, indicating a structural homogeneity between the polycrystalline powder and the single crystal. The crystal structure of salt PPD + ·SUL À revealed a 1:1 stoichiometry and the SUL À anion adopts the sulfa-amine C(8) hydrogen-bond pattern, forming twodimensional supramolecular sheets. Thermal analysis showed the loss of PPD before the melting of SUL.